Re: [PATCH v2 1/1] pgo: Fix allocate_node() v2

Nathan Chancellor
2021-06-03T20:52:46Z

On 6/3/2021 1:50 PM, Nick Desaulniers wrote:
> On Thu, Jun 3, 2021 at 6:41 AM Jarmo Tiitto <jarmo.tiitto@gmail.com> wrote:
>>
>> Based on Kees and others feedback here is v2 patch
>> that clarifies why the current checks in allocate_node()
>> are flawed. I did fair amount of KGDB time on it.
>
> Kees can probably cut it when merging, but the above paragraph might
> be better "below the fold" below next time (doesn't necessitate a v3).
>
>>
>> When clang instrumentation eventually calls allocate_node()
>> the struct llvm_prf_data *p argument tells us from what section
>> we should reserve the vnode: It either points into vmlinux's
>> core __llvm_prf_data section or some loaded module's
>> __llvm_prf_data section.
>>
>> But since we don't have access to corresponding
>> __llvm_prf_vnds section(s) for any module, the function
>> should return just NULL and ignore any profiling attempts
>> from modules for now.
>>
>> Signed-off-by: Jarmo Tiitto <jarmo.tiitto@gmail.com>
>> ---
>
> ^ ie. here. If you put text between the `---` and the diffstat, git
> just discards it when applying. It's a good way to hide commentary
> just meant for reviewers when sending a patch.
>
>
>> kernel/pgo/instrument.c | 21 ++++++++++++---------
>> 1 file changed, 12 insertions(+), 9 deletions(-)
>>
>> diff --git a/kernel/pgo/instrument.c b/kernel/pgo/instrument.c
>> index 0e07ee1b17d9..afe9982b07a3 100644
>> --- a/kernel/pgo/instrument.c
>> +++ b/kernel/pgo/instrument.c
>> @@ -23,6 +23,7 @@
>> #include <linux/export.h>
>> #include <linux/spinlock.h>
>> #include <linux/types.h>
>> +#include <asm-generic/sections.h>
>> #include "pgo.h"
>>
>> /*
>> @@ -55,17 +56,19 @@ void prf_unlock(unsigned long flags)
>> static struct llvm_prf_value_node *allocate_node(struct llvm_prf_data *p,
>> u32 index, u64 value)
>> {
>> - if (&__llvm_prf_vnds_start[current_node + 1] >= __llvm_prf_vnds_end)
>> - return NULL; /* Out of nodes */
>> -
>> - current_node++;
>> -
>> - /* Make sure the node is entirely within the section */
>> - if (&__llvm_prf_vnds_start[current_node] >= __llvm_prf_vnds_end ||
>> - &__llvm_prf_vnds_start[current_node + 1] > __llvm_prf_vnds_end)
>> + const int max_vnds = prf_vnds_count();
>
> Sorry, where was prf_vnds_count() defined? I don't see it in
> linux-next, but I'm also not sure which tree has
> 5d0cda65918279ada060417c5fecb7e86ccb3def.

It is generated via the __DEFINE_PRF_SIZE macro in kernel/pgo/pgo.h.

>> + /* Check that p is within vmlinux __llvm_prf_data section.
>> + * If not, don't allocate since we can't handle modules yet.
>> + */
>> + if (!memory_contains(__llvm_prf_data_start,
>> + __llvm_prf_data_end, p, sizeof(*p)))
>> return NULL;
>>
>> - return &__llvm_prf_vnds_start[current_node];
>> + if (WARN_ON_ONCE(current_node >= max_vnds))
>> + return NULL; /* Out of nodes */
>> +
>> + /* reserve vnode for vmlinux */
>> + return &__llvm_prf_vnds_start[current_node++];
>> }
>>
>> /*
>>
>> base-commit: 5d0cda65918279ada060417c5fecb7e86ccb3def
>> --
>> 2.31.1
>>
>
>
> --
> Thanks,
> ~Nick Desaulniers
>